Outdoor Tech blogger Clyde Soles notes a study which says pack straps decrease circulation in hands and fingers.
Using ultrasound and a pulse oximeter, the scientists measured brachial artery pressure as well as capillary flow in the finger tips. After 10 minutes, only a 26 pound load was sufficient to reduce blood flow in the arm by about 43% and in the finger tips by about 54%. They blame this on the straps compressing the axillary vein. It’s particularly noteworthy for mountaineers since this reduction could make us more prone to frostbite.
Clyde posted a picture of a Sherpa with one of those head straps they wear while carrying huge loads up Mount Everest, and notes that studies done in Africa show people can carry a lot more weight on their heads than they might realize.
I suspect we’re so enamored of shoulder straps because the outdoors world has traditionally taken its cues from the military, where the need to keep one’s head totally mobile for survival reasons obliges shifting pack weight to the shoulders. For just plain old freight hauling where self-defense is less of an issue, the Sherpas have got it down.